A computer network is incomplete without the underlying protocols. Thus, to have a thorough understanding of the computer network, one must understand the protocols and the reference model which forms the basis of communication. In this chapter, we are going to understand the reference model, some background understanding about the need of reference model, and different protocol stack and its purpose in communication. This chapter will give you the understanding of the Open Systems Interconnection reference model and the Transmission Control Protocol/Internet Protocol suite that are important pillars of the computer network and will see the motivations behind this layered approach of protocols.